1. Field of the Invention
This invention relates to spring bows; to centralizing devices using such bows for maintaining a well conduit such as casing in a central position in a wellbore; and to methods for their use.
2. Description of the Prior Art
Centering devices known as "centralizers" have long been used in the oil industry for centering well pipe or casing in a wellbore, particularly in operations for cementing the casing in the wellbore. The most common conventional centralizers have two collars which are connected by and spaced apart by outwardly directed staves, springs or bows which engage and press against the wall of the wellbore.
Several disadvantages are associated with the use of centralizers with springs or bows with large bowed heights. The bowed portion of a spring can be pressed inwardly toward the casing on which the device is emplaced through the application of increasingly greater forces against it; so that with a large bowed height, a comparatively large compressive force is required to deflect the spring bow close to the casing to permit insertion into a wellbore. In starting a centralizer mounted on a casing string, the large height bowed portions are compressed inwardly to a great extent, since the opening into which the casing is inserted usually is not very much greater in diameter than the casing string itself. The bowed portions of the springs, therefore, are subjected to comparatively large forces in compressing them through the greater portion of their deflection toward the casing string, which requires a corresponding large force or load to be imposed downwardly upon the casing string. In many instances, with springs having large bowed heights, the centralizer cannot be started in the opening merely by the weight of the casing itself on which the centralizer is mounted. Often external weight or forces have to be applied on the casing string. The comparatively great force being exerted by the springs on the wall of the opening into which they are inserted creates correspondingly great forces between the springs and the surface casing, which must be overcome in lowering the casing string on which centralizer is mounted, and which may create wear on the springs.
The American Petroleum Institute's Specification 10D provides minimum performance standards, test procedures and marking requirements for casing centralizers. It defines various parameters relating to centralizers as follows:
a. Starting Force: Starting force is the maximum force required to start a centralizer into the previously run casing. The maximum starting force for any centralizer shall be less than the weight of 40 feet (12.2 m) of medium weight casing (see Table 2.1). The maximum starting force is to be determined for a centralizer in new, fully assembled, condition as delivered to the end user, i.e. before the bow springs are subjected to "permanent set." The maximum allowable starting force shall apply to the smallest hole size a centralizer is specified for. PA1 b. Permanent Set: Permanent set is the attainment by the centralizer of a constant bow height less than the original bow height of the bow-springs after repeated flexing of the bow springs. A permanent set is considered established if the bow height remains constant after each spring has been flattened twelve times. The requirement to establish a permanent set of the bow springs before restoring force data are measured simulates the running of the centralizer through bore hole sections. PA1 c. Running Force: Running force is the maximum force required to move a centralizer through the previously run casing. The running force is proportional to and always equal to or less than the starting force. It is a practical value which gives the maximum "running drag" produced by a centralizer in the smallest hole size specified. PA1 Note: Starting and running force values are based on installation of the centralizer per manufacturer's recommendations. Both forces can increase substantially through "wrong" installation (i.e. a close tolerance centralizer which is, contrary to its design, installed over a casing collar). PA1 d. Flattened: Flattened is defined as the point where the springs will not continue to deflect when three times the minimum restoring force is applied to the outer pipe during the starting force and running force tests. PA1 e. Annular Clearance: Annular clearance is the distance between the outside of the casing and the borehole wall when the casing is perfectly centralized. PA1 f. Standoff: For cases where the casing is not centralized, standoff is the smallest distance between the outside of the casing and the borehole wall. PA1 g. Standoff Ratio: The standoff ratio is the ratio of standoff to annular clearance, expressed as a percentage. PA1 For example: PA1 For a 7 inch (178 mm) centralizer run in a 81/2 inch (216 mm) hole, which is intended to maintain a standoff of 0.5 inches (12.7 mm) the standoff ratio is calculated as follows: PA1 Annular clearance= ##EQU1## Standoff to annular clearance ratio ##EQU2## h. Restoring Force: The restoring force is the force exerted by a centralizer against the casing to keep it away from the bore hole wall. The restoring force required from a centralizer, to maintain adequate standoff, is small in a vertical hole, but substantial for the same centralizer in a deviated hole. PA1 Field observations indicate hole deviation on an average varies from zero to approximately 60 degrees: therefore, an average deviation of 30 degrees is used to calculate restoring force requirements, listed in Table 2.1. PA1 For casing sizes 103/4 inches (273 mm) through 20 inches (508 mm), casing strings generally placed in relatively vertical hole sections, the minimum restoring force shall be not less than: EQU RF=W SIN 30=(W/2) PA1 Where: PA1 For casing sizes 41/2 inches (114 mm) through 95/8 inches (244 mm), casing strings generally placed in the deviated hole sections, the minimum restoring force shall be not less than: EQU RF=2W SIN 30=W PA1 The factor of 2 is established as a compensating factor for effect of doglegs. PA1 Due to the applicability of many centralizers in a wise variety of hole sizes, any minimum restoring force specification for centralizer must be based on a standoff value or standoff ratio. A standoff ratio of 0.67 (67 percent standoff) is used in this standard for all minimum restoring force values. PA1 Note: The previously stated ratio of 0.67 is not intended as a specification for adequate centralization of casing in the field, but merely for the purpose of specifying minimum performance standards. Actual restoring force values at various standoff's can be obtained from force-deflection curves as generated from the test procedure contained in Section 4 of this standard. PA1 i. Hole Size Range: The hole size range for which a centralizer meets these specifications and is marked with, in compliance with Section 3, indicates the smallest and largest hole size for which such centralizer meets these specifications. The smallest hole size will be determined by the maximum starting force requirement, the largest by the minimum restoring force requirement.
RF=Minimum restoring force PA2 W=weight of 40 ft. (12.2 m) of medium weight casing
In accordance with 37 C.F.R. .sctn.1.56, submitted herewith are copies of the following references which may be material to this application:
U.S. Pat. No. 2,665,762 discloses a centralizer having springs with comparatively large bowed heights which reduces force by providing a coupling collar for connection with the pin ends of casing, the coupling collar providing a stop member for contacting the upper or lower cage of the centralizer so that when a restriction or tight place is encountered the centralizers bows (springs) are urged inwardly to reduce the force.
U.S. Pat. No. 3,124,196 discloses a centralizer with inclined bows having an arcuate cross-section of sufficiently small radius to present a rounded surface engagement with the wellbore wall.
U.S. Pat. No. 3,566,965 discloses a centralizer with conventional bows and collars formed of a plurality of releasably connected segments which permit tiers of bows to be formed.
U.S. Pat. No. 3,575,239 discloses a centralizer wherein the position of some of its bows is longitudinally offset from other bows whereby less than all of the bows are engaged simultaneously when forcing the device into an opening.
U.S. Pat. No. 4,143,713 discloses a centralizer with its bows held by lugs to keep them engaged in their collars.
U.S. Pat. No. 4,520,869 discloses a centralizer having bows with a "hat" section for positioning in a channel in the collar. Compressing the channel into the bow hat section locks the bow in place.
U.S. Pat. No. 4,531,582 discloses a centralizer with bows having a concave configuration at their apexes, but not near their ends. An L-shaped extension of the collars serves as a restoring force increasing point after the bows have been depressed a certain amount.
German patent application No. P3508086.8-24 discloses a centralizer with axially offset asymmetrical bows. This German application is assigned to a sister company of the assignee of the present application.
U.K. Pat. No. 1,110,840 and Austrian Pat. No. 259,484 disclose a centralizer with collars and bows with a number of U-shaped, outwardly-directed extensions which are received into and secured in the collars. These two patents are assigned to a sister company of the assignee to the present application.
API Spec 10D discloses various general information about centralizers.
B&W Incorporated's 1974-1975 catalog (excerpt) discloses centralizer bows with an arched shape.
"Control Formation Sand," Howard Smith Screen Company, 1982, discloses a variety of conventional centralizers. (See p. 19).
"Primary Cementing Equipment," GEMOCO, 1986 discloses a variety of centralizers and bows.
Weatherford, "Product Information Cementing Aids GmbH," 1985 discloses a variety of prior art bows and centralizers. Weatherford Oil Tool GmbH is a sister company to the assignee of the present application.
Weatherford, "1986-87 Products and Services Catalogs," 1985 (primarily pages 22-28) discloses a variety of prior art bows and centralizers. Weatherford International, Inc. is the parent company of the assignee of the present application.
Generally, the centering force requirement dictates the use of heavy material in the bows, a large number of bows, and a profile with the bows directed outwardly a substantial distant--and yet the centralizer must be inserted into a tubular or bore that is relatively small in circumference. A substantial resistance to insertion is encountered due to, inter alia, the force between the bows and the tubular or bore into which the centralizer is to be inserted.
There has long been a need for a centralizer with a low starting force, yet with a high restoring force.